Process for making electrical contacts of solar batteries and solar batteries made according to this process

ABSTRACT

The invention relates to a process for making solar batteries by means of which a photo-sensitive semi-conductor part is produced and this part provided with two contact areas onto which, according to the invention, contact pieces are fixed by intermetallic diffusion between the two parts to be joined.

United States Patent [1 1 Roger Feb. 19, 1974 [54] PROCESS FOR MAKINGELECTRICAL 3,436,818 4/1969 Merrin et a1 29/589 CONTACTS ()1? SOLARBATTERIES AND 33,420,003 8/1969 Harnpikian 317/234 M ,5 7,508 3/1971 Coxet a1 317/234 M SOLAR BATTERIES MADE ACCORDING 3,651,562 3/1972l-lambleton 317/234 M TO THIS PROCESS Inventor: Jacques Roger, Paris,France Assignee: Societe Anonyme de Telecommunications, Paris, FranceFiled: May 7, 1971 Appl. No.: 141,325

Foreign Application Priority Data Sept. 24, 1970 France 70.34679References Cited UNITED STATES PATENTS 4/1969 Finlayson 29/589 PrimaryExaminer-Roy Lake Assistant Examiner-W. C. Tupman Attorney, Agent, orFirm-Seidel, Gonda & Goldhammer [5 7 ABSTRACT The invention relates to aprocess for making solar batteries by means of which a photo-sensitivesemiconductor part is produced and thispart provided with two contactareas onto which, according to the invention, contact pieces are fixedby intermetallic diffusion between the two parts to be joined.

6 Claims, 2 Drawing Figures Fig.1

PROCESS FOR MAKING ELECTRICAL CONTACTS OF SOLAR BATTERIES AND SOLARBATTERIES MADE ACCORDING TO THIS PROCESS The present invention relatesto a process for making solar batteries, as well as solar batteries madeaccording to this process.

More particularly, the invention relates to a process for making aphoto-sensitive semiconductor part, and providing this part with twocontact areas for collecting electrical energy supplied by the solarbattery.

The development of space research has also resulted in the developmentof techniques related to solar batteries. Thus the power required foroperating a satellite is supplied by a solar generator, consisting of anassembly of batteries or solar cells made, for example, of silicon. Sucha solar generator can cover either the surface of the satellite itselfor the special panels attached to the body of the satellite. Inparticular, when the power required on board is considerable, and thisis increasingly the case, the attached panels that are used are verylarge, about fifty square meters in area, and these must not be unfoldeduntil after the satellite has been launched.

So, to reduce the weight of these panels, silicon cells are fixed nolonger to rigid panels, but onto an elastic support which can be rolledup or folded. The thermal inertia of such a device is reduced, producingthereby extremely steep temperatures gradients, when the panels passfrom the illuminated state to the nonilluminated state, and inversely.

A number of various processes for making solar batteries contacts arealready known. As these batteries are generally used on satellites and,because of this subjected over long periods, sometimes of the order often years, to a very large number of thermal cycles, it has been foundthat such assemblies stand up badly to such conditions. The breakdownsobserved are due in part to the difference in the linear coefficient ofexpansion of the constituents, and in part, to the degradation of thetin normally used in the manufacture of such contacts. In addition, thewelding of the contacts normally used subjects the solar battery to athermal shock which is very localised in the neighborhood of the weld,which affects the functioning of the cell adversely.

The object of the present invention is to overcome these disadvantagesand ,to devise a process enabling the contacts of solar batteries to befixed in a simple manner, without reducing the power supplied by thesebatteries. 7

The present invention therefore relates to a process in which thecontact pieces are fixed onto the contact areas by means ofintermetallic diffusion between the two parts to be joined.

It is particularly advantageous, according to another characteristicfeature of the invention, to effect the intermetallic diffusion duringthe last phase of the heat treatment specific to the battery contacts,this treatment involving the whole of the battery.

According to a variant, the contacts of the battery are produced by atleast one metallic layer submitted to heat treatment in a controlledatmosphere.

In certain cases, for the parts that are in frequent use, the heattreatment is effected by subjecting the parts tobe joined to pressure.

In addition, the invention relates to solar batteries made according tothe process described above. It extends also to the whole of a solargenerator constituted by the assembly of such batteries.

In general, the principle advantage of the present invention lies in thefact that it retains its efficiency in a better way which is veryimportant for solar generators that are put into orbit.

The process will now be described with the aid of a method of makingelectrical contacts of a silicon solar battery, illustrated by anexample represented in the accompanying drawing, in which FIG. 1 showsthe section of a solar battery in three stages A-B-C of manufacture.

FIG. 2 is a perspective view of a completed solar battery, fitted withits contact pieces.

First, the manufacture of a plate of a solar battery made according toknown diffusion processes will be described.

Thus, in FIG. 1A, a plate 1 of silicon P is used and serves as a basefor the manufacture of the solar battery. A layer N, indicated as 2 inFIG. 113, over the entire surface of the plate is obtained by means ofknown diffusion processes.

In the areas where this layer N is not required, it is removed by anyprocess such as is used in engraving, only the upper layer 3 (FIG. 1C)being retained.

The contact areas 4 and 5 are then prepared by the deposition of metalssuch as titanium or silver.

In the perspective view of the completed solar battery, such asillustrated in FIG. 2, there is a silicon plate 1 and the layer that hasbecome N by diffusion and which has not been removed. This layer 3constitutes a network of electrodes terminating in the area 4 providedfor fitting the contact piece 6.

In this FIG. 2, the'lower contact piece 7 is shown fixed to the lowercontact 5 of the solar battery. It is advantageous to select the metalsconstituting, on one hand, the contact areas of the battery, and, on theother, the contact pieces, in a way such that their coefficient ofexpansion is close to that of silicon, in order to avoid to a largedegree the sudden thermal constraints created by the extremely steeptemperature gradients that occur when the batteries pass from theilluminated state to the non-illuminated state, or inversely.

The contact pieces 6 and 7 have a cut-out form, as illustrated as anexample in FIG. 2. The contact pieces can also be in solid form, inwhich case, however, the surface is treated 'so that the adhesionbetween a contact piece and the contact area is obtained only 'at acertain number of points designed to be connection points.

The same result can be obtained with a contact piece that has beentreated completely and a tool with which localised pressure is appliedin order to obtain adhesion of the two metals by means of diffusion.

After the manufacture of the batteries has been completed, checked, andtested electrically, these parts, fitted with integrated contacts areassembled into parallel module series by joining the interconnections bymeans of electric welding.

The assembly can be made either directly or with the help of connectionstrips.

As before, the type of connection strip chosen will depend on theparticular application for which the battery is intended to be used. Forinstance, the coefficient of expansion of this strip can be suited tothat of the substrate, independently of the silicon and the nature ofthe contacts. In addition, the contacts of the solar battery consist ofone or several thin layers of metal, superimposed and subjected to heattreatment in a controlled atmosphere. For this, an interconnection stripmade of meta], between the batteries, is connected directly to thecontact previously coated with layers of one or several metals in order,for example, to improve weldability and avoid corrosion.

To obtain the diffusion of the two external layers into one another andobtain an electrically resistant, permanent joint, it is necessary toselect appropriately the type of the internal layer, the solar contactlayer and that of the contact.

According to one variant, it is possible to insert an intermediate layerbetween the external layers of the battery contact and its connectionlink. The intermediate metal must be chosen so as to obtain a eutecticcombination with the internal layers.

The heat treatment, proper to the battery contact, combined with theheat treatment by means of welding by diffusion, is an essentialcharacteristic feature of the invention.

Many tests have been carried out in application of the presentinvention.

In these tests, in general, a so-called Titanium- Silver contact hasbeen used. Treatment temperatures have ranged between 500C and 700C forperiods of 30 to 5 mn. It has been found advantageous to use pressuresof the order of 50 kg/cm2 to effect the thermal silver-titaniumdiffusion. Under the experimental conditions described above anexcellent low-resistance contact was obtained. The dimensions of thebatteries used in this test were X 20X 0.3 mm. The efficiency of thesebatteries was l0 to '12 percent.

This process enables solar batteries tobe obtained fitted with theirinterconnection strips, and integrated during the last operation in themanufacture of a battery. In this way both the electrical efficiency andreliability of the battery are considerably improved.

It will be obvious that the invention is not limited to the examples ofits applications herein above described and illustrated and that othervariants of it can be envisaged within the scope of the invention.

1 claim:

1. A process for making electrical contacts on a semiconductor solarbattery, said process comprising the steps of:

providing a semiconductor solar battery, said solar battery beingcomprised of a first portion of N type semiconductive material and asecond portion of P type semiconductive material forming aphotosensitive junction portion;

depositing a metal comprised of silver on an electrical contact area onsaid first and second portions of said battery by evaporating said metalin a vacuum onto said contact areas; and

fixing the metal contact pieces to the deposited metal on said contactareas by intermetallic diffusion using temperatures in the range of 500centigrade to 700 centigrade and a pressure of approximately 50kilograms per square centimeter.

2. A process in accordance with claim 1 wherein said depositing stepcomprises the step of depositing a metal comprised of a silver-titaniumalloy.

3. A process in accordance with claim 1 wherein said intermetallicdiffusing is carried out during the last phase of heat treatment in themanufacture of the battery.

4. A solar battery made in accordance with the process of claim 1. y

5. A solar battery in accordance with claim 4 wherein said metaldeposited on said contact area comprises a silver-titanium alloy.

6. A solar battery made in accordance with the process of claim 3.

2. A process in accordance with claim 1 wherein said depositing step comprises the step of depositing a metal comprised of a silver-titanium alloy.
 3. A process in accordance with claim 1 wherein said intermetallic diffusing is carried out during the last phase of heat treatment in the manufacture of the battery.
 4. A solar battery made in accordance with the process of claim
 5. A solar battery in accordance with claim 4 wherein said metal deposited on said contact area comprises a silver-titanium alloy.
 6. A solar battery made in accordance with the process of claim
 3. 